The present invention relates to a method for in situ preparation of siloxane-imide copolymers. More particularly, the present invention relates to a method for the direct preparation of fully imidized siloxane-imide copolymers without first having to form polyamide acid polymers.
It is known in the art that siloxane-imide copolymers can be prepared by reacting, for example, a diaminosiloxane and a dianhydride such as benzophenone dianhydride to yield a polyamic acid having the formula ##STR2## where R is a divalent hydrocarbon radical, R' is a monovalent hydrocarbon radical, m is a whole number greater than one, and n is a whole number greater than 10. Such polyamide acids are soluble in highly polar solvents such as N-methyl pyrrolidone and are provided the end-user in this form.
The silicone polymer is formed, typically after the end-user has applied a coating of polyamide acid to a substrate, by heating at a temperature of from about 150.degree. C. to about 400.degree. C. to remove the solvent and effect cyclization to form a siloxane-imide copolymer, having, for example, the formula ##STR3##
These polyimides, while useful as protective coatings for semiconductors and other electronic devices, suffer from the defect that they are insoluble in virtually all of the common organic acids. Another drawback of methods which require heating the polyamide acid at temperatures of from 150.degree. C. to 400.degree. C. is that many semiconductor devices cannot be heated to such extremes without adversely affecting the device itself. Furthermore, the artisan will appreciate that the amide acid can hydrolyze to form carboxylic acid groups which, of course, will prevent complete imidization when the end-user attempts to use the product. Accordingly, it is desirable to provide a polyimide capable of being applied in the form of an imide rather than in the form of an amide acid.
Holub, U.S. Pat. No. 3,325,450, discloses polyimide-siloxanes of Formula II hereinabove and their preparation by reacting diaminosiloxanes and organic dianhydrides to form a polyamide acid, and thereafter heating the polyamide acid to effect imidization. Variations of Holub's teachings can be found in U.S. Pat. Nos. 3,392,144; 3,435,002; 3,553,282; 3,558,741; 3,663,728; and 3,740,305.
Berger, U.S. Pat. No. 4,011,279, discloses a process for making polyimide-polydiorganosiloxane block copolymers which comprises effecting azeotropic water removal from a refluxing mixture of an organic dianhydride and organic diamine in the presence of an organic solvent and an effective amount of organic acid catalyst where the ratio of organic dianhydride to organic diamine has a value greater than one, allowing the mixture to cool and adding an amount of aminoalkyl terminated polydiorganosiloxane which is substantially equivalent to the excess of organic dianhydride in the previous step, agitating the resulting mixture for a time sufficient to effect polyimide-polydiorganosiloxane block copolymerization.
Berger, U.S. Pat. No. 4,030,948, discloses a polyimide copolymer which is the reaction product of a tetracarboxylic acid dianhydride, an organic diamine and a di(aminoalkyl) polysiloxane, where the di(aminoalkyl) polysiloxane constitutes 18 to 45 mole percent of the total amine requirement of the polymer.
Berger, U.S. Pat. No. 4,395,527, discloses that polyimides containing siloxane of the formula ##STR4## where Q is a substituted or unsubstituted aromatic group; ##STR5## D is an unsubstituted or substituted hydrocarbylene; R.sup.1, R.sup.2, R.sup.3, R.sup.4, R.sup.5, and R.sup.6 are each, independently, unsubstituted or substituted hydrocarbyl radicals; and x, y and z each, independently, have a value from 0 to 100; impart improved solubility and adhesion to the polyimide. Generally, such polyimides are said to be soluble in chlorinated hydrocarbon solvents such as dichlorobenzene and trichlorobenzene, as well as in polar solvents such as N,N-dimethyl acetamide; N-methyl caprolactam; dimethylsulfoxide; N-methyl-2-pyrrolidone; tetramethylurea; pyridine; dimethylsulfone; hexamethylphosphoramide; tetramethylene sulfone; formamide; N-methylforamide; butyrolactone; and N-acetyl-2-pyrrolidone. Berger further teaches that if a diether-containing anhydride is utilized as one of the starting materials, there is obtained a polyimide soluble not only in the chlorinated hydrocarbon solvents and polar solvents previously disclosed, but also, where it contains a siloxane unit, the polyimide is soluble in a solvent which is derived from monoalkyl and/or dialkyl ethers of ethylene glycol and condensed polyethylene glycols and/or cyclic ethers containing no less than a five member ring, such as diglyme. However, Berger makes clear that polyimides will have limited solubility in diglyme. The artisan will appreciate that Berger requires the use of unusual monomers which, accordingly, are rather expensive if it is desired to obtain a diglyme soluble imide-siloxane polymer.